Electric Current and its Effects PDF

10 Electric Current and its Effects Y ou might have tried the game 10.1 SYMBOLS OF ELECTRIC ‘How steady is your hand?’ COMPONENTS suggested in Chapter 9 of Some common electric componen...

10 Electric Current and its Effects Y ou might have tried the game 10.1 SYMBOLS OF ELECTRIC ‘How steady is your hand?’ COMPONENTS suggested in Chapter 9 of Some common electric components can Class VI. If not, you may try it out now. be represented by symbols. In Table 10.1, Paheli and Boojho had also set up the some electric components and their game by connecting an electric circuit symbols are shown. You may come as suggested in Class VI. They had lots across different symbols for these of fun trying it out with their families components in different books. However, and friends. They enjoyed it so much in this book, we shall be using the that they decided to suggest it to a symbols shown here. cousin of theirs who stayed in a Look at the symbols carefully. In the different town. So, Paheli made a neat symbol for the electric cell, notice that drawing showing how the various there is a longer line and a shorter but electric components were to be thicker parallel line. Do you recall that connected (Fig.10.1). an electric cell has a positive terminal and a negative terminal? In the symbol of the electric cell, the longer line represents the positive terminal and the thicker, shorter line represents the negative terminal. For a switch the ‘ON’ position and the ‘OFF’ position are represented by the symbols as shown. The wires used to connect the various components in a circuit are represented by lines. In Table 10.1, a battery and its Fig. 10.1 Setup to check how steady symbol are also shown. Do you know your hand is what a battery is? Look at the symbol of a battery. Can you make out what a Can you draw this circuit battery could be? For some of the conveniently? It made Boojho wonder if activities we may need more than one there was an easier way to represent cell. So, we connect two or more cells these electric components. together as shown in Fig.10.2. Notice Reprint 2024-25  Table 10.1 Symbols for some Many devices such as torches, electric circuit components transistors, toys, TV remote controls, use S.No. Electric component Symbol batteries. However, in some of these devices the electric cells are not always 1. Electric cell placed one after the other as shown in Fig. 10.2. Sometimes the cells are placed side by side. Then how are the terminals of the cells connected? Look carefully inside the battery compartment of any 2. Electric bulb device. There is usually a thick wire or a metal strip connecting the positive terminal of one cell to the negative terminal of the next cell (Fig.10.3). In order to help you to place the cells correctly in the battery compartment, 3. Switch in ‘ON’ position ‘+’ and ‘–’ symbols are usually printed there. How can we connect the cells to prepare batteries for our activities? You may make a cell holder, as shown in 4. Switch in ‘OFF’ position Fig.10.4, using a wooden block, two iron strips and rubber bands. It is necessary (a) (b) 5. Battery Fig. 10.2 (a) A battery of two cells (b) A battery of four cells 6. Wire that the positive terminal of one cell is connected to the negative terminal of the next cell. Such a combination of two or Fig. 10.3 Connecting two cells together to more cells is called a battery. make a battery 110 SCIENCE Reprint 2024-25  terminal of one cell is connected to the negative terminal of the next cell. Connect a piece of wire each to the two metal clips on the cell holder as shown in Fig.10.5. Your battery is ready for use. Fig. 10.4 A cell holder The symbol used for representing a battery is shown in Table.10.1. Let us now draw a circuit diagram of an electric circuit using symbols shown in Table 10.1. Activity 10.1 Make the electric circuit shown in Fig. 10.7. You used a similar circuit in Class VI to make an electric bulb glow. Do you remember that the bulb Fig. 10.5 Another type of cell holder glows only when the switch is in the ‘ON’ position? The bulb glows as soon that the rubber bands hold the metal as the switch is moved to the ‘ON’ strips tightly. position. You could also buy cell holders from Copy this electric circuit in your the market for making batteries of two notebook. Make also a circuit diagram or more electric cells. Place the cells in of this circuit using symbols for the them properly, such that the positive various electric components. Is your diagram similar to the one Paheli and Boojho wonder whether shown in Fig. 10.8? the batteries used in tractors, trucks It is much easier to draw a circuit and inverters are also made from cells. diagram using symbols. Therefore, we Then why is it called a battery? Can generally represent an electric circuit by you help them to find the answer to its circuit diagram. this question? Fig. 10.9 shows another circuit diagram. Is it identical to the circuit diagram shown in Fig.10.8? In which way is it different? Would the bulb glow in this electric circuit? Recall that the bulb glows only when the switch is in the ‘ON’ position Fig. 10.6 Truck battery and its cutout and the electric circuit is closed. ELECTRIC CURRENT AND ITS EFFECTS 111 Reprint 2024-25  Fig. 10.9 Another circuit diagram electric current passes through it. When Fig. 10.7 An electric circuit the bulb gets fused, its filament is broken. CAUTION Never touch a lighted electric bulb connected to the mains. It may be very hot and your hand may get burnt badly. Do not experiment with the electric supply from the mains or a generator or an inverter. You may get Fig. 10.8 Circuit diagram of electric circuit an electric shock, which may be shown in Fig. 10.7 dangerous. Use only electric cells for all the activities suggested here.  Notice that the key or switch can be placed anywhere in the circuit. If the filament of the bulb is broken,  When the switch is in the ‘ON’ would the circuit be complete? Would position, the circuit from the the bulb still glow? positive terminal of the battery to You might have noticed that a the negative terminal is complete. glowing electric bulb become warm. Do The circuit is then said to be closed you know why? and the current flows throughout 10.2 HEATING EFFECT OF ELECTRIC the circuit instantly. CURRENT  When the switch is in the ‘OFF’ position, the circuit is incomplete. Activity 10.2 It is said to be open. No current Take an electric cell, a bulb, a switch flows through any part of the and connecting wires. Make an electric circuit. circuit as shown in Fig.10.9. This In the bulb there is a thin wire, called activity has to be done using only one the filament, which glows when an cell. Keep the switch in the ‘OFF’ 112 SCIENCE Reprint 2024-25 position. Does the bulb glow? Touch The wire gets hot when an electric the bulb. Now move the electric switch current passes through it. This is the to the ‘ON’ position and let the bulb glow heating effect of the electric current. for a minute or so. Again touch the bulb. Can you think of any electric appliance Do you feel any difference? After moving where the heating effect of the electric the switch back to the ‘OFF’ position, current is used? Make a list of such touch the bulb again. appliances. Activity 10.3 Make a circuit as shown in Fig.10.10. Take about 10 cm long piece of nichrome wire and tie it between the nails. (You can get nichrome wire from an electric repair shop or you can use a piece of discarded coil of an electric heater.) Touch the wire. Now switch on the You might have seen an electric room current in the circuit by moving the heater or an electric heater used for switch to the ‘ON’ position. After a few cooking. All these contain a coil of wire. This coil of wire is called an element. You may have noticed that when these appliances are switched on after Boojho could not see element in an electric iron. Paheli told him that electrical appliances, such as immersion heaters, hotplates, irons, geysers, electric kettles, hair dryers, have elements inside them. Have you Fig. 10.10 ever seen the element in any appliance? CAUTION Do not keep the switch in the ‘ON’ position for a long time, otherwise the cell may become weak very quickly. seconds touch the wire. (Do not hold it for a long time.) Switch off the current. Touch the wire again after a few Fig. 10.11 Element of electric iron minutes. ELECTRIC CURRENT AND ITS EFFECTS 113 Reprint 2024-25  connecting to the electric supply, their elements become red hot and give out heat. The amount of heat produced in a wire depends on its material, length and thickness. Thus, for different requirements, the wires of different materials and different lengths and thicknesses are used. The wires used for making electric circuits do not normally become hot. On the other hand, the elements of some electric appliances become so hot that Fig. 10.12 Glowing filament of an electric bulb they are easily visible. The filament of (incandescent) Incandescent electric bulbs (Fig. 10.12) are often used for lighting but they also give heat. It means that a part of electricity consumed is used in producing heat. This is not desirable as it results in the wastage of electricity. The fluorescent tube-lights and compact fluorescent lamps (CFLs) are better electricity efficient lighting sources. Nowadays, the use of light emitting diode (LED) bulbs is increasing. For producing a given intensity of light, LED bulbs consume less electricity as compared to incandescent bulbs or fluorescent tubes or CFLs.Thus LED bulbs are much electricity efficient and therefore being preferred. Fig. 10.13 Electric bulb, tube-light, CFL and LED It is advised to use electrical appliances and gadgets, which are electricity efficient. Bureau of Indian Standards, New Delhi assigns a Standard Mark on products, called ISI mark which is an assurance of conformity to the specifications given on the products. It is therefore suggested to use ISI mark products. Note: Fluorescent tubes and CFLs contain mercury vapour, toxic in nature. Therefore, damaged fluorescent tubes or CFLs need to be disposed off safely. 114 SCIENCE Reprint 2024-25  Fig. 10.15 Fuses used in electrical appliances buildings fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely Fig. 10.14 Fuse used in buildings flow through a circuit. If by accident the an electric bulb gets heated to such a current exceeds this safe limit, the wires high temperature that it starts glowing. If a large current passes through a CAUTION wire, the wire may become so hot that it Never try to investigate an electric fuse may even melt and break. But is it connected to mains circuit on your possible for a wire to melt and break? own. You may, however, visit an Let us check it out. electric repair shop and compare the Activity 10.4 burnt out fuses with the new ones. Make the circuit we used for Activity 10.3 One reason for excessive currents in again. However, replace the cell with a electrical circuits is the direct battery of four cells. Also, in place of touching of wires. This may happen if the nichrome wire, tie a thin strand of the insulation on the wires has come steel wool. (The steel wool is commonly off due to wear and tear. This may used for cleaning utensils and is cause a short circuit. Another reason available in grocery shops.) If there are for excessive current can be the any fans in the room, switch them off. connection of many devices to a single Now pass the current through the circuit socket. This may cause overload in for sometime. Observe the strand of steel the circuit. You might have read wool carefully. Note what happens. Does reports in newspapers about fires the strand of steel wool melt and break? caused by short circuits and Wires made from some special overloads. materials melt quickly and break when may become overheated and may cause large electric currents are passed fire. If a proper fuse is there in the circuit, through them. These wires are used for it will blow off and break the circuit. A making electric fuses (Fig.10.14). In all fuse is thus a safety device which ELECTRIC CURRENT AND ITS EFFECTS 115 Reprint 2024-25  These days Miniature circuit breakers (MCBs) are increasingly being used in place of fuses. These are switches which automatically turn off when current in a circuit exceeds the safe limit. You turn them on and the circuit is once again complete. Look for ISI Fig. 10.16 Miniature mark on MCBs also. Fig. 10.17 Effect of current on a circuit breaker (MCB) compass needle CAUTION wire a few times around the cardboard Always, use proper fuses which have tray. Place a small compass needle been specified for particular inside it. Now connect the free ends of applications, carrying ISI mark. Never this wire to an electric cell through a use just any wire or strip of metal in switch as shown in Fig.10.17. place of a fuse. Note the direction in which the compass needle is pointing. Bring a bar prevents damages to electrical circuits magnet near the compass needle. and possible fires. Observe what happens. Now, while Fuses of different kinds are used watching the compass needle carefully, for different purposes. Fig. 10.14 shows move the switch to the ‘ON’ position. fuses used in our houses. Fuses shown in Fig. 10.15 are generally used in electrical appliances. We observed the heating effect of the electric current and learnt how we use it to our advantage. Does the electric current have other effects also? 10.3 MAGNETIC EFFECT OF ELECTRIC CURRENT Activity 10.5 Take the cardboard tray from inside a Fig. 10.18 Hans Christian Oersted discarded matchbox. Wrap an electric (A.D. 1777-1851) 116 SCIENCE Reprint 2024-25 What do you observe? Does the compass Remember not to switch on the needle deflect? Move the switch back to current for more than a few seconds the ‘OFF’ position. Does the compass at a time. The electromagnet weakens needle come back to its initial position? the cell quickly if left connected. Repeat the experiment a few times. What does this experiment indicate? 10.4 ELECTROMAGNET We know that the needle of a compass is a tiny magnet, which points in Activity 10.6 north-south direction. When we bring Take around 75 cm long piece of a magnet close to it, the needle gets insulated (plastic or cloth covered or deflected. We have also seen that enamelled) flexible wire and an iron nail, compass needle gets deflected when the say about 6–10 cm long. Wind the wire current flows in a nearby wire. Can you tightly around the nail in the form of a connect the two observations? When the coil. Connect the free ends of the wire current flows through a wire, does the to the terminals of a cell through a switch wire behave like a magnet? as shown in Fig. 10.19. This is what a scientist called Hans Place some pins on or near the end Christian Oersted (Fig. 10.18) also of the nail. Now switch on the current. wondered. He was the first person who What happens? Do the pins cling to the noticed the deflection of compass needle tip of the nail? Switch off the current. every time the current was passed Are the pins still clinging to the end of through the wire. the nail? So, when electric current passes The coil in the above activity behaves through a wire, it behaves like a magnet. like a magnet when electric current flows This is the magnetic effect of the electric through it. When the electric current is current. In fact, an electric current can switched off, the coil generally loses its be used to make magnets. Do you find magnetism. Such coils are called it too surprising? Let us try it out. electromagnets. The electromagnets can be made very strong and can lift very heavy loads. Do you remember the crane about which you read in Chapter 10 of Class VI? The end of such a crane has a strong electromagnet attached to it. The electromagnets are also used to separate magnetic material from the junk. Doctors use tiny electromagnets to take out small pieces of magnetic material that have accidentally fallen in Fig. 10.19 An electromagnet ELECTRIC CURRENT AND ITS EFFECTS 117 Reprint 2024-25 the eye. Many toys also have the electromagnet. There is a contact electromagnets inside them. screw near the iron strip. When the iron strip is in contact with the screw, the 10.5 ELECTRIC BELL current flows through the coil which We are quite familiar with an electric bell. becomes an electromagnet. It, then, It has an electromagnet in it. Let us see pulls the iron strip. In the process, the how it works. hammer at the end of the strip strikes Fig. 10.20 shows the circuit of an the gong of the bell to produce a sound. electric bell. It consists of a coil of wire However, when the electromagnet pulls wound on an iron piece. The coil acts the iron strip, it also breaks the circuit. as an electromagnet. An iron strip with The current through the coil stops a hammer at one end is kept close to flowing. Will the coil remain an electromagnet? The coil is no longer an electromagnet. It no longer attracts the iron strip. The iron strip comes back to its original position and touches the contact screw again. This completes the circuit. The current flows in the coil and the hammer strikes the gong again. This process is repeated in quick succession. The hammer strikes the gong every time the circuit is completed. This is how Fig. 10. 20 Circuit of an electric bell the bell rings. Keywords Battery Electric bell Heating effect of current Circuit diagram Electromagnet Magnetic effect of current Electric components Fuse What you have learnt It is convenient to represent electric components by symbols. Using these, an electric circuit can be represented by a circuit diagram. When an electric current flows through a wire, the wire gets heated. It is the heating effect of current. This effect has many applications. 118 SCIENCE Reprint 2024-25  Wires made from some special materials melt quickly and break when large electric currents are passed through them. These materials are used for making electric fuses which prevent fires and damage to electric appliances. When an electric current flows through a wire, it behaves like a magnet. A current carrying coil of an insulated wire wrapped around a piece of iron is called an electromagnet. Electromagnets are used in many devices. Exercises 1. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery 2. Draw the circuit diagram to represent the circuit shown in Fig.10.21. Fig. 10.21 3. Fig.10.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells. Fig. 10.22 4. The bulb in the circuit shown in Fig.10.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow. ELECTRIC CURRENT AND ITS EFFECTS 119 Reprint 2024-25  Fig. 10.23 5. Name any two effects of electric current. 6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain. 7. Will the compass needle show deflection when the switch in the circuit shown by Fig.10.24 is closed? Fig. 10.24 8. Fill in the blanks: (a) Longer line in the symbol for a cell represents its terminal. (b) The combination of two or more cells is called a. (c) When current is switched ‘on’ in a room heater, it. (d) The safety device based on the heating effect of electric current is called a. 9. Mark ‘T’ if the statement is true and ‘F’ if it is false: (a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F) (b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F) (c) An electromagnet does not attract a piece of iron. (T/F) (d) An electric bell has an electromagnet. (T/F) 10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain. 120 SCIENCE Reprint 2024-25  11. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response. 12. Zubeda made an electric circuit using a cell holder shown in Fig. 10.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit. 13. In the circuit shown in Fig. 10.25 A B C Fig. 10.25 (i) Would any of the bulb glow when the switch is in the ‘OFF’ position? (ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position? Extended Learning — Activities and Projects 1. Set up the circuit shown in Fig. 10.17 again. Move the key to ‘ON’ position and watch carefully in which direction the compass needle gets deflected. Switch ‘OFF’ the current. Now keeping rest of the circuit intact, reverse the connections at the terminal of the cell. Again switch ‘on’ the current. Note the direction in which the needle gets deflected. Think of an explanation. Paheli and Boojho saw a magic trick sometime back. The magician placed an iron box on a stand. He then called Boojho and asked him to lift the box. Boojho could easily lift the box. Now the magician made a show of moving his stick around the box while muttering some thing. He again asked Boojho to lift the box. This time Boojho could not even move it. The magician again muttered some thing and now Boojho could lift the box. The audience, including Paheli and Boojho, were very impressed with the show and felt that the magician had some supernatural powers. However, after reading this chapter Paheli is wondering if the trick was indeed some magic or some science was involved? Can you guess what science might be involved? ELECTRIC CURRENT AND ITS EFFECTS 121 Reprint 2024-25  2. Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets. 3. Using an electromagnet, you can make a working model of a railway signal as shown in Fig.10.26. Cardboard signal Thread Iron nail Coil Fig. 10.26 A working model of a railway signal 4. Visit an electric shop. Request an electrician to show you the various types of fuses and MCB and to explain how they work. Did you know? The credit for the invention of the electric bulb is usually given to Thomas Alva Edison, though others before him had worked on it. Edison was a remarkable man. He made some 1300 inventions including the electric bulb, gramophone, the motion picture camera and the carbon transmitter, which facilitated the invention of the telephone. Fig. 10.27 Thomas Alva Edison (A.D. 1847 – 1931) 122 SCIENCE Reprint 2024-25